Colds are contagious illnesses which are clearly stated in your policy. And they are SUPER visable/ easy to spot by the whole colored mucus and having to wipe their nose every 2 minutes. Their infected mucus is smearing all over your daycare and other people's children. And, quite frankly, unless you have a laboratory in your daycare... I'm assuming you're not doing cultures to find out if it's ACTUALLY a cold, or if it's strep, bronchitis, or pertussis. All 3 of which start off looking like a cold, and are often fever free in the first few most contagious days.
Send out a reminder. It can be worded upbeat and nicely and still be very firm. I would also include a bit where if it becomes obvious during the day parents will be receiving a phonecall.
In most cities hospitals offer "sick child daycare". You might even look up to see if yours does and put their contact info on your reminder.
ADDED:
Our preschool had a STRICT "no sick children" policy. In the 3 years we were there it was AMAZING how few times we got sick, and how *QUICKLY* the kids recovered when they were kept home and taken care of. Versus daycares & preschools with lax policies that my friends used. They were sick from October to May... just kind of constantly. The kids in our school, however, might be sick once every other month for a few days. It's true for adults, as well. When we can actually stay home and take care of ourselves we're better in no time. When we keep working and pushing our bodies, however, we're sick for AGES. The immune system needs the body to REST in order to function properly. Every time we force our bodies to switch to the sympathetic nervous system, our immune system (under the parasympathetic nervous system) gets shut off, allowing bacteria and viruses to breed uncontrollably. 2 steps forward, 1 step back... instead of just moving swiftly forward.